1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet, and an image forming apparatus such as a copying machine, a facsimile, a printer and the like, having such a sheet feeding apparatus and adapted to form an image on the sheet fed from the sheet feeding apparatus.
2. Related Background Art
In the past, sheet feeding apparatuses adopted various sheet feeding systems and sheet separating systems. As an example of the sheet separating systems, there is a retard separating system.
Now, a construction and an operation of a sheet feeding apparatus 29 having the retard separating system will be briefly described with reference to FIG. 8.
When a single sheet is fed between a feed roller 8 and a retard roller 9 which are urged against each other, the feed roller 8 is rotated together with a rotary drive shaft 21 in a direction shown by the arrow A. The retard roller 9 is rotatingly driven by rotation of the feed roller 8 via the sheet in a direction shown by the arrow C opposite to a rotational direction B of a rotating rotary drive shaft 24. The both rollers 8, 9 constitute a pair of separation rollers.
The reason that the retard roller 9 provided on the rotary drive shaft 24 rotated in the direction B can be rotatingly driven by the rotation of the feed roller 8 in the direction opposite to the rotational direction of the rotary drive shaft 24 is presence of a torque limiter 25 disposed between the retard roller 9 and the rotary drive shaft 24 so that reverse rotation of the retard roller is allowed by the torque limiter 25.
If a plurality of sheets are double-fed, a frictional force of the torque limiter overcomes a frictional force between the sheets, so that the retard roller 9 is rotated in the direction B.
Thus, only the sheet contacted with the feed roller 8 is fed, and the other sheets are not fed by the retard roller 9, thereby separating the other from the fed sheet.
In this way, since the sheet feeding apparatus 29 having the retard separating system can separate the sheet positively, it has high reliability and has been widely used.
However, in the sheet feeding apparatus 29, a sheet separating ability may be worsened and/or sheet jam may occur due to wear and/or deformation of rubber portions 8a, 9a of the pair of separation rollers 8, 9. Thus, the pair of separation rollers 8, 9 must be designed so that they can easily be replaced by new ones by an end user.
To this end, the pair of separation rollers 8, 9 are designed so that they are detachably mounted on one ends of the rotary drive shafts 21, 24 protruded from a support 20 and an arm 18 to be able to be detached by a one-touch operation, thereby facilitating the replacement of the rollers. That is to say, although the feed roller 8 and the retard roller 9 are normally prevented from disengaging from the rotary drive shafts 21, 24 by engagement between circumferential engagement grooves 4, 5 formed in one ends of the rotary drive shafts 21, 24 and engagement projections 8d, 9d provided on the feed roller 8 and the retard roller 9, by disengaging the engagement projections 8d, 9d from the engagement grooves 4, 5, the rollers 8, 9 can easily be removed from the rotary drive shafts 21, 24.
However, in the above-mentioned sheet feeding apparatus 29, if tolerances of parts are added or accumulated or if any part is worn due to long term use, as shown in FIG. 9, the rotary drive shafts 21, 24 may be inclined to offset the retard roller 9 from the feed roller 8.
Further, since the pair of separation rollers 8, 9 are provided on the ends of the rotary drive shafts 21, 24, the rotary drive shafts 21, 24 are flexed more or less, with the result that the retard roller 9 may be offset from the feed roller 8.
Incidentally, in FIG. 9, although it is exaggerated so that a gap between the rollers 8 and 9 is not uniform completely from left to right, actually, any gap is not generated due to elasticity of the rubber portions 8a, 9a, but, it generally seems as if the rollers are uniformly contacted along the entire length thereof. Acordingly, even when it seems that there is no gap between the rollers 8 and 9, an urging force between the rollers 8, 9 is not always uniform.
If the rollers 8, 9 are offset from each other, the rubber portions 8a, 9a of the rollers 8, 9 will be worn eccentrically to generate a thrust load directing toward a left-and-right direction in FIG. 9, thereby increasing load torque, with the result that the rotary drive shafts 21, 24 cannot be rotated smoothly.
As a result, the sheet separating ability of the sheet feeding apparatus 29 is worsened. Further, if the rubber portions 8a, 9a of the rollers 8, 9 are worn eccentrically, service lives of the rollers will be shortened, with the result that the rollers 8, 9 must be replaced frequently.
Therefore, it is important that parallelism between the feed roller 8 and the retard roller 9 be maintained or ensured, and, there is a need for providing substantially uniform urging load (radial load) between the rollers through the entire length thereof.
Further, in an image forming apparatus having such a sheet feeding apparatus for feeding a sheet in this way, since the sheet cannot be fed positively and correctly, an image cannot be formed on the sheet correctly.